If the principal area moments of the suspension beams of a tuning fork gyro are not aligned with the drive force, drive force or motion will result in sense axis (Y axis) motion which will be interpreted as input rate. This can be reduced by dynamically balancing the device.
Traditionally, dynamic balancing of macro-sized translating or rotating structures have involved the physical attachment of masses, such as metal pieces on the rims of automobile tires, by mechanical fastening or adhesive techniques. These procedures are impractical when dealing with miniaturized vibrating structures.
In miniaturized-scale devices, the focus has been mostly on controlling the resonant frequency, as with the quartz crystal tuning forks in the watch industry. The timekeeping mechanism in a quartz watch is the quartz crystal, shaped in the form of a conventional tuning fork. Quartz watches are very stable because the resonant frequencies of the crystals are extremely stable. Parts costing less than a dollar each, experience typical frequency shifts of about a few parts in 107 over the course of a month. In production, the as-fabricated resonant frequencies of the quartz crystals are somewhat variable due to inadequate process control. The small frequency deviations from the norm are corrected later by laser ablating controlled amounts of metal (typically gold) that is pre-deposited on the tips of the tuning fork tines. Only the mass of the tines is affected; little change in geometry and the stiffness of the vibrating structure results from this operation. The change in mass manifests itself as a change in the tuning fork""s resonant frequency.
Others have dynamically balanced quartz tuning fork gyros through the tedious process of having an operator place small epoxy dabs as masses on the vibrating members. The process is neither practical for the order of magnitude smaller silicon microstructures nor cost effective.
According to the teaching of the present invention a tuning fork gyro is balanced in the true dynamic sense, even as the tuning forks continue to vibrate. Real time balancing and quadrature bias reduction are, thus, achieved.
The present tuning fork gyro, as shown for example in copending and commonly owned U.S. Pat. Nos.: 5,349,855; 5,388,458; 5,496,436; 5,767,404; 5,892,153; 5,952,574, in a preferred embodiment consists of two silicon mass structures suspended with two folded beam structures. The flexures ensure that the tuning fork anti-parallel mode is excited and that the translational modes are attenuated. The combs are excited so that electrostatic forces are generated which only weakly depend on the lateral position of the masses. The resulting large amplitude vibrations, parallel to the teeth, increase gyro sensitivity and reduce errors from external forces such as Brownian motion. When the device is rotated about its input axis (which is coincident with the sense axis and is in the plane of the substrate), Coriolis forces push one mass up and the other down. Capacitor plates below the proof masses are used for sensing this displacement. The differential capacitance measured for the two proof masses is related to the input angular rate.
The gyro output signal, proportional to input rate, is in-phase with the velocity of [2{right arrow over (xcfx89)}xc3x97{right arrow over (xcexd)}] the proof masses. The Coriolis acceleration, where xcfx89 is the input rotation rate and v is the velocity, and, thus, the Coriolis force is a maximum where the velocity is the maximum. The maximum velocity position during operation corresponds with the neutral at-rest position of the tines in their unexcited state. With the tines resonating, the out-of plane displacements from angular rate inputs are a maximum at this position. Constant input rate results in vertical proof mass motion and sense axis differential capacitance at the drive frequency in-phase with the drive velocity. The differential capacitance and the output voltage also contain signals in phase with the drive position and in quadrature with the desired input rate information. The major source of quadrature signal is proof mass vertical motion caused by the suspension beams"" principal area moment of inertia not being aligned with the drive forces. Since gyroscopes are generally operated at resonance in vacuum, the spring forces and quadrature signals are larger than the forces to drive the proof mass.
The quadrature bias signal has been typically quite large; it is generally considerably larger than the in-phase gyro bias signal. The magnitude and stability of the quadrature bias are both of concern because they corrupt the measurement of the actual input rate. The aim of the balancing task is to reduce the quadrature bias signal to a level, below the in-phase bias signal.
This is achieved by the use of laser ablation or deposition in a substantially vacuum environment, including the vacuum packaged finished gyro. Additionally, material that has been laser melted can be moved to affect balancing. The ablation deposition, or moving of material can occur during operation, typically with the vibrating elements at maximum velocity at the near neutral position rather than at motional extremes. The material ablation or addition occurs in the support structures for the device rather than on the proof masses, because the effect of changes are greater on the quadrature signal at those points.